A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. It is convenient for areas in close commercial or other communication to keep the same time, so time zones tend to follow the boundaries of countries and their subdivisions.
Most of the time zones on land are offset from Coordinated Universal Time (“UTC”) by a whole number of hours (UTC−12 to UTC+14), but a few are offset by 30 or 45 minutes. Some higher latitude countries use daylight saving time for part of the year, typically by changing clocks by an hour. Many land time zones are skewed toward the west of the corresponding nautical time zones. This also creates a permanent daylight saving time effect.
Many countries, and sometimes just certain regions of countries, adopt daylight saving time (“DST”, also known as “Summer Time”) during part of the year. This typically involves advancing clocks by an hour near the start of spring and adjusting back in autumn (“spring” forward, “fall” back). Modern DST was first proposed in 1907 and was in widespread use in 1916 as a wartime measure aimed at conserving coal. Despite controversy, many countries have used it off and on since then; details vary by location and change occasionally. Most countries around the equator do not observe daylight saving time, since the seasonal difference in sunlight is minimal.
When storing times that may be subject to daylight saving time, the standard time format may be used. Standard time is the result of synchronizing clocks in different geographical locations within a time zone to the same time rather than using the local meridian as in local mean time or solar time. Historically, this helped in the process of weather forecasting and train travel. The concept became established in the late 19th century. The time so set has come to be defined in terms of offsets from Universal Time. Where daylight saving time is used, the term “standard time” typically refers to the time without the offset for daylight saving time.
A government can change the time zone definition at its discretion at any time. A typical example is the expansion of daylight saving time in the U.S. in 2007. In 2006 and earlier, daylight saving time started in the U.S. on the first Sunday in April and ended on the last Sunday in October. Starting in 2007, the daylight saving period has been expanded to start on the second Sunday in March and end on the first Sunday in November. The time of the change remained the same at 2 AM.
For example, it is in the year 2006 and a calendar application has an appointment scheduled for Oct. 31, 2007. The system is based on the rule definition as of 2006 and the scheduled appointment is in the standard time because the rule definition is based on the rules prior to the change in 2007. After the rule change comes into effect, the calendar application may behave unexpectedly because the scheduled time on Oct. 31, 2007 points to an incorrect moment, one hour behind, as it is misinterpreted to be in the standard time.